1. Field of the Invention
This invention relates to continuously variable transmissions (CVTs) in which the belt connecting the sheave system of the CVT is protected against slippage caused by an excessive torque. More particularly, this invention relates to the location of a non-dynamic clutch downstream of the belt and sheave that is configured for slippage at high torques, prior to the slippage of the belt.
2. Description of the Prior Art
A continuously variable transmission system utilizes a pair of adjustable pulleys, including a primary pulley and a secondary pulley. Each pulley has at least one sheave that is axially fixed and another sheave that is axially movable relative to the first. A flexible belt of metal or elastomeric material interconnects the pulleys. The inner faces of the sheaves of the pulley are bevelled or chamfered so that as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus, the effective pulley diameter may be adjusted. The primary pulley is driven by an engine and the output of the secondary pulley drives the drive train of the vehicle. The drive train is typically connected to the secondary pulley through a clutch. U.S. Pat. No. 4,433,594, entitled "Variable Pulley Transmission", provides further information regarding CVTs and is incorporated herein in terms of background to the present invention.
The displaceable sheave of each pulley includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. Increasing the fluid in the chamber increases the effective diameter of the pulley. As fluid is exhausted from the chamber, the pulley diameter is decreased. Generally, the effective diameter of the primary pulley is moved in one direction as the effective diameter of the secondary pulley is moved in the other.
The movement of the sheave of the primary pulley servo regulates the transmission ratio across the CVT. The hydraulic pressure of the sheave of the secondary pulley servo varies the clamping force on the belt connecting the primary and secondary pulleys. It is important to control the clamping force on the belt connecting the two pulleys to prevent damage that might result from belt slippage. Slip will occur if the sheave clamping force is inadequate to transmit the torque supplied by the engine across the continuously variable transmission. This slippage is the occurrence with which this invention is primarily concerned.
A torque spike is likely to occur whenever the vehicle drive wheels slip and then reengage. When the drive wheels slip, the transmission output shaft continues its rotation in the direction in which it is being driven by the engine. However, when the drive wheels stop slipping and are reengaged, there is resistance to the rotation of the drive shaft and a torque spike is suddenly experienced by the CVT. This sudden torque spike will overload the belt connecting the primary and secondary pulleys and cause the belt to slip. Such slippage occurs on account of the belt clamping force being only marginally above that required to prevent slippage for normal driving conditions.
There are several situations in which a torque spike might cause belt slippage. One such situation is when a car skids on an icy patch of road. As the car skids, the wheels rotate freely. When the skid is corrected, the wheels grip the road surface and a momentary reaction torque is experienced. A similar sequence of events occurs when the drive wheels of a vehicle momentarily disengage from the ground, as for example, when passing over a pot hole. Serious damage can occur to the belt and sheaves of a CVT if no precautions are taken and the momentary reaction torque causes the belt of the belt and sheave system to slip.
One approach to the prevention of damage caused by a torque spike can be found in U.S. Pat. No. 4,433,594, mentioned previously. In this system, a slippable start clutch is positioned downstream of the belt and sheave system. The start clutch is arranged to slip when the torque experienced by the clutch is above a critical value. The use of a slippable clutch with a variable clutch capacity requires a control system to ensure that the torque at which the clutch slips is at all times compatible with system requirements. The continuously variable transmission system of the present invention provides a rather more compact arrangement for a continuously variable transmission system utilizing economy of space.